1. Field of Invention
The present invention relates to the technical field of an electro-optical device such as a liquid-crystal display device, and a method for manufacturing the electro-optical device. More particularly, the present invention relates to the technical field on an electro-optical device and a method for manufacturing the electro-optical device, which is appropriate for use in liquid-crystal display devices employing a TN (Twisted Nematic) liquid crystal, in particular, a thin-film transistor (hereinafter referred as TFT) active-matrix liquid-crystal display device, which adopts an alternating drive method in which the polarities of the potentials applied to adjacent pixel electrodes are periodically alternated every pixel row or every pixel column, so that the potentials applied to adjacent pixel electrodes in a row direction or in a column direction are inverted in polarity.
2. Description of Related Art
Electro-optical devices, such as liquid-crystal display devices, typically include an electro-optical material such as a liquid crystal interposed between a pair of substrates, and the alignment state of the electro-optical material is controlled by the property of the electro-optical material and an alignment layer formed on the substrate on its surface facing the electro-optical material. If there is a step in the surface of the alignment layer, an orientation defect occurs in the electro-optical material depending on the magnitude of the step. If such an orientation defect occurs, proper driving of the electro-optical material in that portion becomes difficult, and the contrast ratio of the device drops due to a visible defect in the electro-optical device. Since a TFT active-matrix electro-optical device includes, on a TFT array substrate, TFTs in many locations thereof for controlling and switching a variety of lines such as scanning lines, data lines, and capacitance lines, and includes pixel electrodes, a step inevitably occurs in the surface of an alignment layer in accordance with the presence of lines and elements, if no planarizing process is performed.
Conventionally, the portion of the substrate suffering from such a step is aligned with the spacing between adjacent pixel electrodes, and a light-shielding layer covers the portion of the step so that the portion of the electro-optical material suffering from the orientation defect may remain hidden or may not contribute to display light.
The electro-optical device of this sort typically adopts an alternating drive method in which the polarity of a potential applied to the pixel electrodes is alternated at a predetermined pattern to prevent degray scale of the electro-optical material as a result of the application of a direct current, and to control cross talk and flickering of a display screen image.
A 1H alternating drive method is relatively easy to control and presents a high-quality image display. During the presentation of a video signal of one frame or one field, the pixel electrodes arranged on an odd row are driven by a positive potential, while the pixel electrodes arranged on an even row are driven by a negative potential. During the presentation of a video signal of a next frame or a next field, conversely, the pixel electrodes arranged on the even row are driven by a positive potential while the pixel electrodes arranged on the odd row are driven by a negative potential, and at the same time, the potential polarity is alternated every row in the period of frame or field.
A 1S alternating drive method is also easy to control and presents a high-quality image display. The pixel electrodes on the same column are driven by the same polarity potential while the potential polarity is alternated every column in the period of frame or field.
The technique to cover the above-referenced step portion with the light-shielding layer narrows the aperture of the pixel depending on the size of the step portion, and cannot meet the basic requirement in the technical field of the electro-optical device that the aperture ratio of the pixel be increased in a limited image display area to present a brighter image. The number of lines and TFTs per unit area increases as the pixel pitch becomes fine for high-definition video presentation. Since there is a limitation to the miniaturization of the lines and the TFTs, the ratio of the step portion to the image display area becomes relatively high, and the problem of the step portion becomes serious as high-definition design is promoted in the electro-optical device.
In accordance with the above-referenced technique for planarizing the interlayer insulator beneath the pixel electrodes, no particular problem will be presented when adjacent pixel electrodes are of the same potential in a TFT array substrate. When the potentials (the potentials applied to the pixel electrodes adjacent in the column direction in the 1H alternating drive method, and the potentials applied to the pixel electrodes adjacent in the row direction in the 1S alternating drive method) are opposite in polarity as in the above-referenced 1H alternating drive method or 1S alternating drive method, the gap between the pixel electrode and the counter electrode becomes wider at the edge of the pixel electrode over the line and the TFT when the planarizing process is performed than when no planarizing process is performed. A transverse electric field taking place between the adjacent pixel electrodes (specifically, an electric field in parallel with the surface of the substrate or slant electric field having a component in parallel with the surface of the substrate) relatively intensifies. If such a transverse electric field is applied to the electro-optical material, which is expected to work under a longitudinal electric field present between the pixel electrodes and the counter electrode (i.e., an electric field perpendicular to the surface of the substrate), a disclination takes place in the electro-optical material, visible defect occurs there, and the contrast ratio drops. Although the area of the transverse electric field can be covered with the light-shielding layer, the aperture area of the pixel is reduced along with an increase in the area of the transverse electric field. As the distance between the adjacent pixel electrodes shrinks with a fine pixel pitch, the transverse electric field intensifies, and these become more problematic as high-definition design is promoted in the electro-optical device.
The present invention has been developed in view of at least the above problems. It is an object of the present invention to provide an electro-optical device, such as a liquid-crystal display device, which presents a high aperture ratio of pixel and displays a high-contrast-ratio, bright and high-quality image by controlling an orientation defect resulting from a step portion in the surface of a substrate in contact with an electro-optical material, such as a liquid crystal, while by keeping the aperture area of each pixel from narrowing as much as possible.
An electro-optical device of a first exemplary embodiment of the present invention includes a first substrate having a first alignment layer that has been subjected to a rubbing process; a second substrate opposed to the first substrate, having a second alignment layer that has been subjected to a rubbing process; an electro-optical material interposed between the first substrate and the second substrate; a step portion, formed on at least one of the first alignment layer of the first substrate and the second alignment layer of the second substrate, and downwardly rubbed in the direction of the rubbing process; and a light-shielding layer formed in an area facing the step portion that is downwardly rubbed in the direction of the rubbing process, on at least one of the first substrate and the second substrate.
The study carried out by the inventors of this invention reveals that an orientation defect of an electro-optical material due to a step is substantially more pronounced when a rubbing process is performed in a downward direction than when a rubbing process is performed in an upward direction. Specifically, a relatively good orientation is expected in an upwardly rubbed portion regardless of a step, while the step causes a substantial orientation defect in a downwardly rubbed portion. This is attributed to the fact that upwardly rubbing and flat-surface rubbing actions tend to result in similar interactions between an alignment layer which results from the rubbing process and defines an alignment state of the electro-optical material and the electro-optical material, while downwardly rubbing and flat-surface rubbing actions tend to result in unsimilar interactions.
The area facing the step portion that is downwardly rubbed is light-shielded by the light-shielding layer in the present invention. Although an orientation defect takes place in the electro-optical material in the downwardly rubbed portion, the downwardly rubbed portion is light-shielded in an non-aperture area of each pixel, and no visible defect occurs. In other words, a drop in contrast ratio due to the orientation defect is prevented by light-shielding the downwardly rubbed portion.
To cover an orientation defect in the electro-optical material due to the step in the downwardly rubbed portion, the light-shielding layer is preferably set to be slightly wider in width than the downwardly rubbed portion.
The step portion may be at a projection that is formed to extend in a direction intersecting the direction of the rubbing process.
The projection is preferably formed in an area that corresponds to a spacing between adjacent pixel electrodes that are driven in mutually different polarities.
With this arrangement, a longitudinal electric field on the projection is intensified, while a transverse electric field taking place between pixel electrodes is weakened.
An electro-optical device such as a matrix driving liquid-crystal display device may use a 1H alternating drive method and a 1S alternating drive method as an alternating drive manner.
Preferably, the upwardly rubbed portion of the projection which is upwardly rubbed in the direction of the rubbing process is not opposed to the light-shielding layer.
The upwardly rubbed portion is a portion that contributes to presentation without causing a visible defect, and the aperture ratio of pixel is increased without lowering the contrast ratio by leaving the upwardly rubbed portion unshielded as much as possible.
The step portion may be at a hollow portion that is formed to extend in a direction intersecting the direction of the rubbing process.
The hollow portion may be a groove formed in one of the first substrate and the second substrate, and a line may be arranged in the area of the groove.
The upwardly rubbed portion of the hollow portion which is upwardly rubbed in the direction of the rubbing process is not opposed to the light-shielding layer, and the aperture ratio of pixel is thus increased without lowering the contrast ratio.
An area of the substrate corresponding to the spacing between adjacent pixel electrodes which are driven in the same polarity is preferably subjected to a planarizing process.
Almost no orientation defect occurs in the electro-optical material due to the step portion between the pixel electrodes by the planarization. To light-shield the area, a narrow light-shielding layer works. The pixel aperture ratio is even more increased.
The planarizing process may be performed by forming a groove in the substrate and arranging a line in the area of the groove.
The line may be a data line of a light-shielding film fabricated of Al (aluminum), for instance, and in this area, a light-shielding feature is imparted to the data line.
The distance between the adjacent pixel electrodes which are driven in the same polarity is preferably larger than the layer thickness of the electro-optical material.
This arrangement controls in the electro-optical material the disclination which is generated by the transverse electric field.
The direction of the rubbing process may be perpendicular to the downwardly rubbed portion of the step portion, or slant to the downwardly rubbed portion of the step portion.
An electro-optical device of a second exemplary embodiment of the present invention includes a first substrate having a first alignment layer that has been subjected to a rubbing process; a second substrate opposed to the first substrate, having a second alignment layer that has been subjected to a rubbing process; a liquid crystal interposed between the first substrate and the second substrate, a portion of the liquid crystal having a reverse tilt angle, and formed on the surface of at least one of the first alignment layer of the first substrate and the second alignment layer of the second substrate; and a light-shielding layer formed in an area facing the portion of the liquid crystal having the reverse tilt angle, on at least one of the first substrate and the second substrate.
The electro-optical device of this exemplary embodiment of the present invention increases the pixel aperture ratio without lowering the contrast ratio, by light-shielding the portion of the liquid crystal having the reverse tilt angle.
An electro-optical device of a third exemplary embodiment of the present invention includes a first substrate having a plurality of pixel electrodes and a first alignment layer that has been subjected to a rubbing process; a second substrate opposed to the first substrate, having a counter electrode and a second alignment layer that has been subjected to a rubbing process; an electro-optical material interposed between the first substrate and the second substrate; a step portion formed on the surface of the first alignment layer in an area corresponding to a spacing between the pixel electrodes on the first substrate, and downwardly rubbed in the direction of the rubbing process; and a light-shielding layer formed in an area facing the step portion that is downwardly rubbed in the direction of the rubbing process, on at least one of the first substrate and the second substrate.
An electro-optical device of a fourth exemplary embodiment of exemplary embodiment of the present invention includes a first substrate having a plurality of pixel electrodes and a first alignment layer that has been subjected to a rubbing process; a second substrate opposed to the first substrate, having a counter electrode and a second alignment layer that has been subjected to a rubbing process; an electro-optical material interposed between the first substrate and the second substrate; a light-shielding layer formed at least on one of the first substrate and the second substrate and defining a pixel area; and a step portion having an upwardly rubbed portion in the direction of the rubbing process, the upwardly rubbed portion being formed in the vicinity of the area facing the light-shielding layer on the surface of the first alignment layer on the first substrate.
The electro-optical device of this exemplary embodiment of the present invention increases the pixel aperture ratio without lowering the contrast ratio, by leaving the upwardly rubbed portion unshielded as much as possible.
An electro-optical device of a fifth exemplary embodiment of the present invention includes a first substrate, formed of a plurality of layers including a first alignment layer that has been subjected to a rubbing process and having a plurality of pixel electrodes; a second substrate opposed to the first substrate, having a counter electrode and a second alignment layer that has been subjected to a rubbing process; an electro-optical material interposed between the first substrate and the second substrate; a groove formed on the first substrate; a line arranged along the groove; a step portion formed on the surface of the alignment layer in the area of the groove; and a light-shielding layer formed in an area facing the step portion that is downwardly rubbed, at least on one of the first substrate and the second substrate, wherein the direction of the rubbing process of the first substrate is aligned in a downwardly rubbing direction to the step portion.
In this exemplary embodiment, the line may form a storage capacitor.
An electro-optical device of a sixth exemplary embodiment of the present invention includes a first substrate having a plurality of pixel electrodes; a second substrate opposed to the first substrate; an electro-optical material interposed between the first substrate and the second substrate; a projection formed on the surface of the alignment layer of first substrate in an area corresponding to the spacing between adjacent pixel electrodes that are driven in mutually different polarities, the projection including a downwardly rubbed portion in the direction of the rubbing process of the first substrate; and a light-shielding layer formed in an area facing the downwardly rubbed portion, on at least one of the first substrate and the second substrate.
A substrate having a plurality of pixel electrodes of another exemplary embodiment of the present invention includes an alignment layer that is subjected to a rubbing process, and a step portion formed on the surface of the alignment layer in an area corresponding to a spacing between the pixel electrodes, and upwardly rubbed in the direction of the rubbing process.
The step portion may be formed at a groove in which a line is arranged, or at a projection for weakening a transverse electric field taking place between pixel electrodes.
The downwardly rubbed portion of the projection is preferably light-shielded by the light-shielding layer.
A method for manufacturing an electro-optical device of another exemplary embodiment of the present invention, wherein the device includes mutually opposed first substrate and second substrate with an electro-optical material interposed therebetween, a plurality of pixel electrodes and an alignment layer formed on the first substrate, and a counter electrode opposed to the pixel electrodes and formed on the second substrate, includes forming a substrate surface in one direction in which pixel electrodes are adjacent to each other so that the alignment layer between the pixel electrodes and the alignment layer over the pixel electrodes are planarized; forming, in the other direction in which pixel electrodes are adjacent to each other, a first step portion projecting from the substrate surface beneath the spacing between the adjacent pixel electrodes; forming the pixel electrode so that the edge of the pixel electrode comes to the first step portion; performing a rubbing process to the alignment layer; and forming a light-shielding layer on at least one of the first substrate and the second substrate so that the light-shielding layer overlaps one of the inclined surfaces of the first step portion in a plan view, along which the direction of the rubbing process of the alignment layer is downward.
A method for manufacturing an electro-optical device of another exemplary embodiment of the present invention wherein the device includes mutually opposed first and second substrates with an electro-optical material interposed therebetween, a plurality of pixel electrodes and an alignment layer formed on the first substrate, and a counter electrode opposed to the pixel electrodes and formed on the second substrate, the plurality of the pixel electrodes composed of a first group of pixel electrodes which are alternately driven in a first period, and a second group of pixel electrodes which are alternately driven in a second period complementary to the first period, includes forming a substrate surface in one direction in which pixel electrodes of the same pixel electrode group are adjacent to each other so that the alignment layer between the pixel electrodes of the same pixel electrode group and the alignment layer over the pixel electrodes are planarized; forming a first step portion projecting from the substrate surface beneath the spacing between a pixel electrode of the first pixel electrode group and a pixel electrode of the second pixel electrode group adjacent to the first group pixel electrode; forming the pixel electrode so that the edge of the pixel electrode comes to the first step portion; performing a rubbing process to the alignment layer; and forming a light-shielding layer on at least one of the first substrate and the second substrate so that the light-shielding layer overlaps one of the inclined surfaces of the first step portion in a plan view, along which the direction of the rubbing process of the alignment layer is downward.
The electro-optical device is thus manufactured through the above exemplary manufacturing methods.
The operation and other advantages of the present invention will be apparent from the following discussion of the exemplary embodiments.